Hitherto, a stepping motor of this kind is constituted in a state that plural stator yokes having plural pole teeth formed by press process are placed in a manner that their respective pole teeth interleave to each other with a uniform interval, that is, with a position of 180.degree. lagging in the electric angle, or are lagged in a manner that they are mutually facing back to back as much as the step angle, that is, in a manner that they are 90.degree. lagging in the electric angle.
However, it is difficult to constitute a stator inner radius part which positions with a small gaps with a rotor periphery at its respective pole teeth by combining plural stator yokes; and since bearings for supporting a rotor shaft are held by side plates fixed to stator yokes which position at both ends of the stator, the centering of its two bearings is also difficult.
Consequently, in stepping motors of prior art, defects in making steps such as touching between rotor and stator or twisting of a rotor shaft have happened often.
For such the constitution of prior art, a configuration such as shown in FIG. 1 was proposed, as is known by a Japanese Unexamined Published Application No. Sho 59-63972.
In the configuration shown in FIG. 1, since a stator 31 is monolithically moulded by resin with using the inner radius of stator yokes 32 and 33 as its reference, a centering process between the inner radius part 31a of the stator 31 and a bearing 34 can be achieved easily. And by the similar reason, since a welding process can be removed from the assembly process, defects such as locking of a rotor or anomalous sounding due to welding spatter are distinguished, and thereby it becomes possible to offer high quality motors with less process defect. Moreover, since parts can be added one by one to a stator which is monolithically moulded with resin, that is, since being different from a building-up of combinations of tentatively assembled parts on intermediate stages, the raw materials are directly assembled into a motor, auxiliary components such as tapes and tubes needed for protecting and holding coil winding wires and others become unnecessary, making the automation of assembly process easier.
On the other hand of such the advantage, however, in the configuration of FIG. 1, on the outer radius part of a stator 31 which is set into a frame 36 having a bearing 35, center deviations in the inner and outer radii included in individual components of stator yokes 32 and 33 take place. That is, the reference has been set on the inner radii of the stator yokes 32 and 33, all of their deviations appear on the outer radius of the stator 31. Consequently, there have been problems that the centering of the bearings 34 and 35 as well as the maintenance of a small gap between the rotor 37 and the inner radius of the stator become difficult.
Also there has been a problem that the separated windings for respective phases of coil windings 38 and 39 which are separated by separators 31b of a stator coil winding part introduce an increase of the wire winding process time.